running 2 steam accounts

i dont think they would ban you for that.

Ask steam support 2 b sure

As much as I don't support doing this, you could try downloading the .gcf or whatever files separately, like off some site, and then allowing your brother to play. Once your brother's done, move the respective files into place, verify game cache, try it. I haven't really tried it myself, but it worked for some people (apparently Steam kept downloading a corrupted file).

It isn't against their rules, you wont be banned for it.

It sounds like you're getting upset that your game has to update (Is this what you meant by 'my game wont install'?) when you're not on your account and taking it out on your brother. Seems like it could all be avoided with a little more maturity and the realization that IT'S JUST A GAME.

Not sure if this have been said but you can run 2 steam clients if you use sandbox. Sandboxie is a program you can use for this.

My liberal solution is to do your downloading overnight.

Sandboxie is like a simplified version of VM (Virtual Machine).
Running a program in VM is like running a program in a totally different machine (using the same hardware, hence 'virtual').

Running Steam in 2 'different' machine is fine and doesn't violate anything. You should be fine.

Or, if you are paranoid, you can do this:
1- Create a new Windows user account, login there.
2- Download your game.
3- Switch user (Don't Log Off), and let your brother play his game. Your Windows account's Steam will continue to download, while your brother's Steam is playing game.

I STRONGLY recommend making 2 different steam folders else i for see problems, this will how ever take more hd space. Also add the steam folders and .exe to Sandbox settings->Resource Access->Direct Access

No problem.
I've been using it for several months now to trade between my side accs and main, and also to idle, lately.

You won't get into any trubs.

I'd recommend using normal Steam to run the most active account though, since the account running through Sandboxie is more limited due to restrictions on the Sandbox that sometimes bitch around.

http://www.facepunch.com/threads/1102531 (screw clickies!) some settings there if that link still works (I got weird internet right now), ignore the stuff that isn't general to a Steam Sandboxie.

Note that the agreement refers to something else, but allows the use of virtual machines to multi-acc. (I don't know how it is possible otherwise anywho, but I can imagine some ways it might be that Valve apparantly does not agree with)

Also, Sandboxie is a bitch on 64-bit sometimes.

Steam says that they don't condone 2 steam installations on same PC, meaning having Steam in C:\program files and D:\program files, an application would probably have conflicting issues if it's installed twice on same OS. You can't login using both accounts at same time if they're in same steam installation, you prolly can with boxie so if that works for you, I see no issue, but if something weird does happen then it's on your head as they warned you they don't support running Steam twice on same pc.

Honestly, easiest thing to do is to keep your downloads done overnight. That way, you can play, your brother can play and you won't have to fight over who's doing what. Also less slow interwebs while you're using the computer since during the day you won't be installing anything.

I simply use virtual box. I never heard of sandboxie but virtual box is a virtual machine so enough said ;)